Alternate provider factory

[ppython]

This patch provide an optional alternative to the current core.py/provider_factory method by setting an extra variable in the settings.

You could customize where the PAYMENTS_VARIANTS settings should be (DB instead of settings for exemple); have as many "stripe settings", isolated from one another.

I tried to provide as well a doc with this patch "doc/alternate_provider_factory.rst", it's a draft and unlinked from the other, as-is.

I needed to be able to configure a 'per entity in the DB' PAYMENT_VARIANTS list through the backend of my Django project. This patch helped me resolve this problem.

[codecov-io]

Current coverage is 78.13% (diff: 28.57%)

Merging #121 into master will decrease coverage by 0.21%

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[esistgut]

This is a feature I really need, any progress on the merge?

[patrys]

No progress here to report. We're thinking about an entirely different API for a future release of django-payments to better support other use cases.

[ppython]

Excellent, though do you have an estimation of the target release date? weeks/months? (to keep track)

[WhyNotHugo]

I know it's been years, but I'd love to see this happen.

Would an updated PR for this be accepted?